Effects of Seedling Protectors and Weed Control on Blue Oak Growth and Survival1

Two factors limiting successful recruitment of planted blue oak (Quercus douglasii) seedlings are dry soils and animal damage. Many approaches have been used to mitigate these factors including several types of protective cages and alternative methods of weed control. This study examined how treeshelters, screen cages and varying intensities of weed removal affect the establishment and growth of blue oak seedlings. After 5 years, seedlings protected with treeshelters had higher survival, greater diameter, and were taller than those in screen cages. Seedlings receiving no weed control had lower survival, shorter height, and smaller diameter than those with the two highest intensities of weed removal. This study suggests that treeshelters are a promising tool for regenerating blue oaks in California and that providing adequate weed control can improve the growth and survival of planted seedlings. Blue oak (Quercus douglasii) is one of several species of native California oaks that is reported to be regenerating poorly in some locations (Bolsinger 1988, Muick and Bartolome 1987). Recent studies indicate that a variety of factors limit natural recruitment, including herbivory by deer, livestock, and rodents; defoliation by insects; root clipping by gophers; girdling by voles; and limited soil moisture induced by competing vegetation. These same factors can also prevent successful establishment of planted seedlings or acorns (McCreary 1990). Many devices are used to protect planted seedlings from damage by animals, with varying degrees of success. Some of the more common products are plastic mesh cages, cylinders made from aluminum window screen, cages of chicken wire, and hardwire cloth buried in the ground. One relatively new product is called a treeshelter. These are rigid, translucent, double-walled plastic cylinders, developed in England and used there for more than 10 years (Potter 1988). They are reported to not only protect seedlings from a variety of animals, but also stimulate aboveground growth. Another factor that can severely limit the survival and growth of oak seedlings on many rangeland sites is severe competition from grasses and forbs. Such competition can create extremely dry soil conditions that can be lethal to both natural seedlings and planted oaks (Griffin 1971). Some researchers believe that plant competition on many hardwood rangelands is greater today than it was before the introduction and establishment of exotic Mediterranean annuals, which have displaced many native perennials (Welker and Menke 1987). These annual plants absorb more soil moisture in the spring than the native perennial grasses, and consequently, create a drier environment. This makes it more difficult for oak seedlings to become established. Because of the adverse effects of such competition, researchers and practitioners have found controlling weeds around oak plantings necessary to obtain adequate survival and growth. Without weed control, animal damage problems are also generally greater, because dense weeds provide a favorable habitat for animals such as grasshoppers and voles, which can seriously damage young plants (Tecklin and McCreary 1993). A variety of techniques have been used to eliminate weeds, including mulch, herbicides, scalping and mowing. Though studies comparing weed control to no weed control have demonstrated the advantage of weed removal (Adams and McDougald 1995), we are aware of no research on the effectiveness of varying intensities of weed removal on the field performance of blue oak seedlings. 1 An abbreviated version of this paper was presented at the Symposium on Oak Woodlands: Ecology, Management, and Urban Interface Issues, March 19-22, 1996, San Luis Obispo, Calif. 2 Natural Resources Specialist and Staff Research Associate, respectively, with the Department of Environmental Science, Policy and Management, University of California, Berkeley. Mailing address: 8279 Scott Forbes Road, Browns Valley, CA 95918.